Metformin is a widely used biguanide diabetes drug that is associated with decreased breast cancer risk and is currently being studied for treatment and prevention of breast cancer. While metformin and biguanides buformin and phenformin exhibit inhibitory activity against breast cancer they lack potency and their mechanisms of action are unclear. Accordingly, there is a need for more potent agents, including biguanides that treat breast cancer including estrogen positive breast cancer including estrogen positive HER2 negative breast cancer (ER+ HER2−).